While alternating current (AC) power is used for transferring power from a location where the power is generated to a remote load, many, if not most, loads other than electric motors require power at a direct current (DC), substantially constant voltage. Therefore, many of these potential loads require a dedicated power converter of a design that can accommodate their respective power requirements.
Local generation of power from renewable resources such as solar and wind turbine power generators has also favored local distribution of power as direct current at high voltages and currents. Some types of loads such as electrically powered vehicles may require large currents at high voltages that exceed the capacities of commercially available or economically feasible components such as switches which can be operated more efficiently than analog circuits and thus are generally favored. In switched converters for high voltage applications, three-level power converters are favored as reducing voltage stress on switches and allowing use of smaller passive components since three-level converters essentially provide two switched power converters such that their output voltages are summed. To meet high current requirements, it is common to provide two or more power converters in parallel. Such parallel-connected power converters operated as a single power converter are generally referred to as multi-phase power converters and, while any number of phases or “legs” may be provided (e.g. ten or more), two to four parallel-connected power converters are most common.
It is possible to drive the individual parallel-connected power converters of a multi-phase power converter in phase with each other. However, doing so results in a substantial output ripple voltage and input and output ripple current, requiring large input and output capacitances. For that reason, it is much more common to provide switching operations in the respective legs which is out-of-phase by a particular angular difference. More specifically, it is more common to control switching such that the output current of each phase has a (usually equal) phase difference so that the sum of the output currents will have a higher equivalent frequency and smaller ripple. Such operation is referred to as interleaving.
However, when the legs of a multi-phase, three-level power converter are operated in an interleaved manner for output current ripple and output capacitor size reduction, the parallel connection allows an abnormally large circulating current ripple in the inductor current which is larger than the output current ripple. This circulating ripple current is a source of resistive losses in the inductors and other circuitry, increased turn-off switching losses and requires a larger inductor to maintain a given level of input and output ripple. Therefore, use of interleaved operation of a power converter to reduce input and output ripple has compromised both efficiency and power density as well as cost and weight.
A three-level power converter, whether or not operated in an interleaved manner, may also cause common mode (CM) noise to be developed at the switching frequency, requiring attenuation by a filtering arrangement to prevent CM noise from being reflected back to the power source. Interleaved operation will reduce the CM noise somewhat but causes high circulating current which is not tolerable for practical use. Since the frequency of the CM noise is at the switching frequency which is relatively low to limit switching losses, the CM filter required for attenuation at the switching frequency is therefore usually relatively large and a significant fraction of the total volume of the power converter, which, in turn, tends to limit the power density of the power converter design.
Since high power density of a power converter is a very desirable attribute, sophisticated designs have been developed to increase power density to the point of principally being a function of the limitations of output ripple current, circulating ripple current and common mode noise specifications. Thus, in high power applications, passive components are a major factor affecting power converter system costs, volume and weight while input and output ripple specifications are becoming more stringent in some applications such as photovoltaic (PV) cell arrays for solar power generation where ripple causes deleterious effects on the PV cells.